Structure of the airflow above surface waves

Department: University of Delaware, School of Marine Science and Policy

Publisher: University of Delaware

Date Issued: 2015

Abstract: We present a laboratory investigation of the structure of the turbulent airflow
above water surface gravity waves. Specifically, we investigate the intimate coupling of
the wind with the waves, and we examine the role of the turbulent airflow kinematics
for the momentum flux across the air-water interface.
The airflow dynamics above the air-sea interface are believed to have a significant
impact on the fluxes of momentum and scalars across the ocean surface. We present
an experimental study of the turbulent structure of the airflow above waves, including
instantaneous, mean, and wave-phase-averaged airflow characteristics. Measurements,
taken at a fetch of 22.7 m in University of Delaware’s large wind-wave-current facility,
are reported. We present results for a total of 17 different wind-wave conditions,
including 5 wind wave experiments with 10-m extrapolated wind speeds spanning from
2.19 m s−1 to 16.63 m s−1. By combining winds with mechanically generated swells,
we were able to achieve a wide range of wave ages Cp/u∗, from 1.4 to 66.7, where Cp is
the peak wave phase speed, and u∗ the air friction velocity. In order to complete this
study, we developed a complex imaging system, combining particle image velocimetry
with laser induced fluorescence techniques. High resolution two-dimensional (18.7 x
9.7 cm2) velocity fields were measured as close as 100 μm above the air-water interface
(on average). In addition, we acquired high spatial and temporal resolution wave field
data simultaneously with the airflow measurements.
The mean velocity profile follows the law of the wall in low winds (U10 = 0.86 m
s−1, no waves detected). Over wind waves, the aerodynamic roughness of the airflow
increases with increasing wind speed. Using our imaging system, we were able to
measure airflow velocities within the viscous sublayer of the airflow boundary layer.
Viscous sublayers remain intact and coherent upwind of wave crests at least up to a moderate wind speed of U10 = 9.41 m s−1. We were able to measure two-dimensional
near-surface spanwise vorticity fields in the airflow. We observe direct evidence of
airflow separation events past the crests of wind waves, starting at low to moderate
wind speeds (U10 > 2.19 m s−1). With increasing wind speed, the contribution of
viscous stress to total wind stress decreases exponentially (in favor of form drag), and
the frequency of airflow separation events increases. At high wind speeds (U10 = 16.63
m s−1), over 85% of the waves experience airflow separation. Airflow separation causes
dramatic along-wave variations in viscous stress.
In all 17 experiments, the turbulent boundary layer in the air is characterized
by numerous velocity sweeps and ejections, accompanied by intense downwind-tilted
spanwise vorticity (shear) layers stemming from the surface. We were able to directly
observe these turbulent events, and estimate their statistical significance using quadrant
analysis. These events become phase-locked in the presence of waves, and over
young wind waves (Cp/u∗ < 3.7), they are replaced by intermittent airflow separation
events past wave crests. The mean airflow is subjected to a sheltering effect past wave
crests, above the critical height c (defined by hu ( c)i = Cp). Mean along-wave turbulent
momentum and energy in the airflow are also phase-locked. Intermittent airflow
separation events past young wave crests cause free high shear layers to generate on
average intense turbulence downwind of crests. We observe an opposite, upwind sheltering
effect below the critical height. The airflow within the critical layer is strongly
coupled with the wave orbital velocities at the water surface, starting at relatively low
wave ages (Cp/u∗= 6.5).
Preliminary instantaneous field measurements are consistent with our laboratory
results.